The present invention relates generally to a method of sensing crankshaft rotational position.
It is well known in the art that the resistance modulation of magnetoresistors can be employed in position and speed sensors with respect to moving magnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456). In such applications, the magnetoresistor (MR) is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object rotating relative and in close proximity to the MR, such as a toothed wheel, produces a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The MR will have a higher magnetic flux density and a higher resistance when a tooth of the rotating target wheel is adjacent to the MR than when a slot of the rotating target wheel is adjacent to the MR. The use of a constant current excitation source provides an output voltage from the MR that varies as the resistance of the MR varies.
Increasingly more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing precise position information during cranking. Various combinations of magnetoresistors and single and dual track toothed or slotted wheels (also known as encoder wheels and target wheels) have been used to obtain this information (see for example U.S. Pat. Nos. 5,570,016, 5,731,702, and 5,754,042).
The electronic control module (ECM) of an engine specifies the required format of the crankshaft position signal. Invariably, the target wheel (i.e., encoder) is designed to generate a magnetic signal conforming to the format of the required signal. That is, preferably, the target wheel will have teeth at crank angles where the position signal should have a high value and slots at crank angles where the position signal should have a low value. The position sensor should convert the mechanical pattern of the target wheel, as closely as possible, into a corresponding electrical signal.
FIG. 1A is a schematic representation of an exemplar automotive environment of use according to this prior art scheme, wherein a target wheel 410 is rotating about an axis 410xe2x80x2, such as for example in unison with a crankshaft, a drive shaft or a cam shaft, and the rotative position thereof is to be sensed. Rotative position of the target wheel 410 is determined by sensing the passage of a tooth edge 412, either a rising tooth edge 412a or a falling tooth edge 412b, using a differential MR sequential sensor 50. A tooth edge 412 is considered rising or falling depending upon the direction of rotation of the target wheel 410 with respect to the magnetoresistive sensors MR1 and MR2. MR1 is considered leading and MR2 is considered lagging if the target wheel 410 is rotating in a clockwise (CW) direction whereas if the target wheel is rotating in a counterclockwise (CCW) direction then MR1 is considered lagging whereas MR2 is considered leading. For purposes of example, the target wheel 410 will be assumed to be rotating in a CW direction in the views.
The differential MR sequential sensor 50 employs two magnetoresistor elements, MR1 and MR2, which are biased by a permanent magnet 56, wherein the magnetic flux 418 and 420 emanating therefrom are represented by the dashed arrows. The magnetic flux 418 and 420 pass from the permanent magnet 56 through the magnetoresistors MR1 and MR2 and through the air gaps 422 and 424 to the target wheel 410. The target wheel 410 is made of a magnetic material having teeth 426 and spacings 428 therebetween and the sensor signal VS is available between terminals 430 and 432.
The example of the target wheel 410 in FIG. 1A is a 3X target wheel. This target wheel 410 and the associated sensor 50 utilize analog signals, available between terminals 430 and 432, which are converted into a 3 bit digital signal that is repeated every 360 degrees of rotation of the wheel. The ideal, error free, situation is depicted by the digital signal in FIG. 1B wherein each bit 426xe2x80x2 represents a particular angular position of the target wheel 410 and adjacent bits are angularly separated by 120 degrees representing the tooth pattern 426 of the target wheel and the desired signal pattern whereby the rising edges 412a of the teeth occur at the rising edges of the signal 412xe2x80x2a and the falling edges 412b of the teeth occur at the falling edges of the signal 412xe2x80x2b. 
However, the actual digital signal is depicted in FIG. 1C wherein each bit 426xe2x80x3 represents a particular angular position of the target wheel 410 and adjacent bits are not angularly separated by 120 degrees due to an angular position error E whereby the rising edges 412a of the teeth 426 do not occur at the rising edges of the signal 412xe2x80x3a and an angular position error Exe2x80x2 whereby the falling edges 412b of the teeth do not occur at the falling edges of the signal 412xe2x80x3b. The angular position errors E and Exe2x80x2 are caused by graduality of change of magnetic field at approach and recession of the teeth, which is sometimes compensated by making the teeth narrower. Another component of the error is caused by variations in the air gaps 422 and 424 as well as variations in temperature.
Another target wheel of interest is the 24X target wheel (see for example U.S. Pat. No. 5,570,016). This wheel and its associated sensor utilize analog signals which are converted into a 24 bit digital signal that is repeated every 360 degrees of rotation of the wheel. Each bit represents a particular position of the wheel and adjacent bits are angularly separated by 15 degrees. In general, target wheels of interest may be specified as nX target wheels where n is an integer number of teeth or slots. These wheels and their associated sensors utilize analog signals which are converted into an n bit digital signal that is repeated every 360 degrees of rotation of the wheel. Each bit represents a particular position of the wheel and adjacent bits are angularly separated by (360/n) degrees. Prior art uses of these wheels have utilized sensors incorporating two matched MRs with a more costly dual track wheel when high accuracy was required or with less expensive single track wheels when less accuracy was acceptable.
What is needed is a method and apparatus to accurately locate the rising and falling edges of the teeth of a single track target wheel whereby the position of the crankshaft can be obtained very accurately and inexpensively.
The present invention provides a method of emulating any desired tooth/slot format of a desired target wheel from a predetermined tooth/slot arrangement of a rotating actual target wheel used in conjunction with an MR position sensor.
According to the method of the present invention, the passage of two sequential slots of differing widths or two sequential teeth of differing widths of the actual target wheel determine the rising and falling edge of one tooth of the desired target wheel and define one tooth and one slot of the desired target wheel. The actual target wheel, has, preferably, 2n teeth and 2n slots of two distinct sequential widths whereby the desired target wheel is emulated to have n teeth and n slots.
For example, an actual target wheel having 6 teeth and 6 slots of two distinct sequential widths, can be used to emulate a 3X desired target wheel with an accuracy attainable previously with a two track target wheel; or, for another example, a actual target wheel having 24 teeth and 24 slots of two distinct sequential widths can be used to emulate a 12X desired target wheel.
The two MRs of the position sensor, are matched, having matched magnetic biasing and powered by matched current sources, and are aligned in the circumferential direction of the actual target wheel so as to generate two angularly offset signals (first and second voltages, respectively) from the passage of a single slot of the actual target wheel. The offset signals are input to a signal conditioning circuit. Within the signal conditioning circuit, the two sensor signals (first and second voltages) are differentially amplified to produce a differential signal whereby the width of the slot is used to encode a binary position voltage, high or low. For example, a wide slot may be a low voltage and encoded as a binary xe2x80x9c0xe2x80x9d while a narrow slot may be a high voltage and encoded as a binary xe2x80x9c1xe2x80x9d although the reverse binary assignments could also be used. These binary assignments (the high and low outputs) of the conditioning circuit reliably identify the rising and falling edges of the teeth of the desired target wheel. That is, two distinct sequential slot/tooth widths of the actual target wheel are used to identify a tooth edge of the desired target wheel as rising or falling and, hence, define the teeth and slots of the desired target wheel. Upon detection of a slot of the actual target wheel, the signal conditioning circuit determines the location of the slot center and whether the slot represents a rising edge or falling edge of a tooth of the desired target wheel and then sets its output voltage respectively high or low and thereby emulate the format of the desired target wheel.
For example, a narrow slot of the actual target wheel is used to identify a tooth edge of the desired target wheel as rising and upon determining the location of the slot center of the narrow slot of the actual target wheel the signal conditioning circuit sets its output voltage high, for example, to thereby denote the rising edge of a tooth of the desired target wheel and define a first edge of a tooth and corresponding end of a slot of the desired target wheel. Subsequently, after the passage of a tooth of the actual target wheel, the next slot of the actual target wheel would be a wide slot and is used to identify a tooth edge of the desired target wheel as falling, for example. Upon determining the location of the slot center of the wide slot of the actual target wheel, the signal conditioning circuit sets its output voltage low to denote the falling edge of a tooth of the desired target wheel and thereby define a second edge of a tooth and corresponding beginning of a slot of the desired target wheel. Subsequently, after the passage of the next tooth of the actual target wheel, the following slot of the actual target wheel would be another narrow slot and the above mentioned process repeats.
Alternatively, the present invention could be implemented with width encoded teeth instead of width encoded slots.
Accordingly, it is an object of the present invention to provide a method for improved position sensing of a rotating article.